The present invention relates to a semiconductor device and a method for manufacturing the same.
One of efficient means for improving performance of semiconductor devices is to enhance a mobility of electrons. In usually-used single crystal silicon, an upper limit of the electron mobility is fixed at a predetermined value. However, it has been reported that the mobility of electrons is enhanced in the crystal silicon having strain in comparison with the normal (strain-free) silicon crystal.
For example, U.S. Pat. No. 5,461,243 discloses a technique for forming a relaxed SiGe layer and a Si layer having strain on a SOI substrate.
However, the aforementioned technique requires an expensive SOI substrate, so that the manufacturing cost inevitably increases.
An object of the present invention is to provide a semiconductor device and a method of manufacturing the same, capable of using a usually-employed silicon substrate and obtaining a crystal silicon layer having strain (a crystal silicon layer being strained) in a good quality.
A method of manufacturing a semiconductor device according to the first aspect of the present invention comprises the steps of:
forming a first crystal silicon layer doped with oxygen on a single crystal silicon substrate; and
forming a crystal silicon-germanium layer (single crystal is preferable) on the first crystal silicon layer.
It is preferable that the method of manufacturing a semiconductor device further comprise steps of forming a second crystal silicon layer (single crystal is preferable) on the crystal silicon-germanium layer; and imparting strain to the second crystal silicon layer by a thermal treatment.
Before the heat treatment is performed, the first crystal silicon layer, the crystal silicon-germanium layer, and the second crystal silicon layer substantially follow lattice information of the underlying single crystal silicon substrate, so that the crystal silicon-germanium layer has strain. When a thermal treatment is applied to such a structure, oxygen contained in the first crystal silicon layer is condensed to form a silicon oxide layer between the silicon substrate and the crystal silicon-germanium layer. As a result, the strain of the crystal silicon-germanium layer is relaxed, in other words, comes to a lattice-relaxed state. Simultaneously, the second crystal silicon layer is formed in a crystalline state having strain.
By virtue of such a function, it is possible to obtain not only the single crystal silicon-germanium layer sufficiently relaxed but also a good single crystal silicon layer having strain, even if the single crystal silicon-germanium layer is thin in film thickness.
A concentration of oxygen contained in the first crystal silicon layer is preferably 1% or more in terms of atomic composition ratio. It is preferable that the uppermost limit of the oxygen concentration be set at a value enough to maintain characteristics as a semiconductor. When the oxygen concentration is higher than 20%, the crystallinity of the first crystal silicon layer is maintained. However, roughness of the surface of the first crystal silicon layer becomes outstanding, degrading a flatness of the surface of the first crystal silicon layer. Therefore, the oxygen concentration of the first crystal silicon layer is preferably set at 20% or less. To improve the flatness, the concentration of oxygen contained in the first crystal silicon layer is preferably set at 15% or less, more preferably 12% or less.
A semiconductor device according to a second aspect of the present invention comprises:
a first crystal silicon layer doped with oxygen and formed on the a single crystal silicon substrate;
a crystal silicon-germanium layer (single crystal is preferable) formed on the first crystal silicon layer; and
a second crystal silicon layer (single crystal is preferable) having strain and formed on the crystal silicon-germanium layer.
A semiconductor device according to a third aspect of the present invention comprises:
a first crystal silicon layer formed on a single crystal silicon substrate, the first crystal silicon layer having a structure in which first crystal silicon regions doped with oxygen sandwich a second crystal silicon region doped with an n-type or p-type impurity;
a crystal silicon-germanium layer (single crystal is preferable) formed on the first crystal silicon layer; and
a second crystal silicon layer (single crystal is preferable) having strain and formed on the crystal silicon-germanium layer.
In the inventions according to the second and third aspects, the crystal silicon-germanium layer is formed on the first crystal silicon layer doped with oxygen, so that dislocation for relaxing the strain of the crystal silicon-germanium layer can be absorbed by the first crystal silicon layer. Therefore, even if the film thickness of the crystal silicon-germanium layer is thin to some extent, it is possible to suppress the dislocation from reaching the crystal silicon-germanium layer. Therefore, it is possible to obtain a crystal silicon-germanium layer sufficiently relaxed and a crystal silicon layer having strain in a good quality.
In addition, since the band gap of the first crystal silicon layer doped with oxygen is wide, it is possible to reduce the capacitance of an element in the same way as in the SOI structure.
Furthermore, in the invention of the third aspect, a potential of the underlying portion can be fixed by the second crystal silicon region doped with an impurity, it is therefore possible to prevent a short-channel effect, efficiently.
As to the oxygen concentration (atomic composition ratio) contained in the first crystal silicon layer, the same limitation as in the first-aspect invention is employed.
A semiconductor device according to the fourth invention comprises
a first crystal silicon layer doped with oxygen and formed on the a single crystal silicon substrate;
a laminated silicon layer formed on the first crystal silicon layer, the laminated silicon layer having a structure in which first crystal silicon regions containing oxygen and second crystal silicon regions undoped with oxygen are alternately stacked;
a crystal silicon-germanium layer (single crystal is preferable) formed on the laminated silicon layer; and
a second crystal silicon layer (single crystal is preferable) having strain and formed on the crystal silicon-germanium layer.
In the fourth invention, an n-type or p-type impurity may be doped in at least one of the second crystal silicon regions.
In the fourth invention, germanium may be doped in at least one of the second crystal silicon regions.
In the fourth invention, the crystal silicon-germanium layer is formed on the laminated silicon layer, so that it is possible to absorb dislocation for relaxing strain of the crystal silicon-germanium layer by the laminated silicon layer having a super lattice structure. Therefore, even if the film thickness of the crystal silicon-germanium layer is thin to some extent, it is possible to suppress the dislocation from reaching the crystal silicon-germanium layer. Therefore, it is possible to obtain a crystal silicon-germanium layer sufficiently relaxed and a crystal silicon layer having strain in a good quality.
Furthermore, when the n-type or p-type impurity is doped in the second crystal silicon region, a potential of the underlying portion can be fixed by the second crystal silicon region. It is therefore possible to prevent a short-channel effect, efficiently.
As to the oxygen concentration (atomic composition ratio) contained in the first crystal silicon layer, the same limitation as in the first-aspect invention is employed.
The present invention provides methods A and B for manufacturing a semiconductor device as described below.
The manufacturing method A comprises the steps of
forming a crystal silicon-germanium layer above a single crystal silicon substrate; and
performing a thermal treatment in an oxidizing atmosphere, thereby
forming a first silicon oxide layer being formed between the single crystal silicon substrate and the crystal silicon-germanium layer and a second silicon oxide layer on the crystal silicon-germanium layer, and reducing the thickness of the crystal silicon-germanium layer and increasing the germanium concentration of the crystal silicon-germanium layer.
When the thermal treatment is performed in the oxidizing atmosphere in the method A, a silicon oxide layer or a silicon layer doped with oxygen is preferably formed between the single crystal silicon substrate and the crystal silicon-germanium layer.
When the thermal treatment is performed in the oxidizing atmosphere, the first and second silicon oxide layers are formed (when the silicon oxide layer is already present before the thermal treatment is performed, the first and second silicon oxide layers are increased in thickness). The reason why the silicon oxide layer is formed not only on the crystal silicon-germanium layer but also between the single crystal silicon substrate and the crystal silicon-germanium layer is that oxygen is transported through the crystal silicon-germanium layer by the thermal treatment performed in the oxidizing atmosphere.
Since the first and second silicon oxide layers are formed, the film thickness of the crystal silicon-germanium layer is reduced. Germanium is rarely present in the first and second silicon oxide layers, is and the first and second silicon oxide layers function as a barrier for suppressing diffusion of germanium. For these reasons, the germanium concentration of the crystal silicon-germanium layer sandwiched by the first and second silicon oxide layers, increases. As a result, it is possible to form a high-quality crystal silicon-germanium layer reduced in thickness and high in germanium concentration, on the silicon oxide layer.
The method B comprises the steps of
forming a crystal silicon-germanium layer on a single crystal silicon substrate; and
performing a thermal treatment in an oxidizing atmosphere, thereby forming a first silicon oxide layer in the crystal silicon-germanium layer and simultaneously forming a second silicon oxide layer on the crystal silicon-germanium layer, and increasing a germanium concentration of the crystal silicon-germanium layer between the first silicon oxide layer and the second silicon oxide layer.
When the thermal treatment is performed in the oxidizing atmosphere in the manufacturing method B, it is preferable that the silicon oxide layer or the silicon layer doped with oxygen be formed in the crystal silicon-germanium layer.
In the manufacturing method B, it is possible to form a high-quality crystal silicon-germanium layer reduced in thickness and high in germanium concentration, on the silicon oxide layer, in the same manner as in the manufacturing method A.
The methods A and B further comprises the steps of
exposing the surface of the crystal silicon-germanium layer by removing the second silicon oxide layer; and
forming a crystal silicon layer having strain on the exposed crystal silicon-germanium layer.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.